I have tried many options both in Mac and in Ubuntu. I read the Rserve documentation
http://rforge.net/Rserve/doc.html
and that for the Rse
If you are not able to shut it down within R, run the codes below to kill it in terminal. These codes work on Mac.
$ ps ax | grep Rserve # get active Rserve sessions
You will see outputs like below. 29155 is job id of the active Rserve session.
29155 /Users/userid/Library/R/3.5/library/Rserve/libs/Rserve
38562 0:00.00 grep Rserve
Then run
$ kill 29155